Shukan Gendai magazine last month sounded a warning: "Students, if you're arrested in China it's a very serious matter."

China is popular with Japanese tourists. Some 2.68 million visited in 2017. It's near — four hours by plane — and yet far: a different world. The spring travel season is here. Students celebrating their graduation — from school, from childhood — flock to China. They see the sights, dine, shop, have "experiences" and come home all the richer for having seen something of life beyond their immediate horizons.

Why should they be arrested?